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Individual differences |
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The following mammals migrate
As is true for many species of deer, especially those in mountainous regions, elk migrate into areas of higher altitude in the spring, following the retreating snows, and the opposite direction in the fall. Hunting pressure also impacts migration and movements. During the winter, they favor wooded areas and sheltered valleys for protection from the wind and availability of tree bark to eat. Roosevelt elk are generally non-migratory due to less seasonal variability of food sources.
The Greater Yellowstone Ecosystem elk herd numbers over 200,000 individuals and during the spring and fall, they take part in the longest elk migration in the continental U.S. Elk in the southern regions of Yellowstone National Park and in the surrounding National Forests migrate south towards the town of Jackson, Wyoming where they winter for up to six months on the National Elk Refuge. Conservationists there ensure the herd is well fed during the harsh winters. Many of the elk that reside in the northern sections of the Greater Yellowstone Ecosystem migrate to lower altitudes in Montana, mainly to the north and west.
Pinnipeds have an amphibious lifestyle; they spend most of the time at sea but haul-out to mate, raise young, molt, rest, thermoregulate or escape from aquatic predators. Several species are known to migrate vast distances, particularly in response to extreme environmental changes like El Niño and changes in ice cover. Elephant seals stay at sea 8–10 months a year and migrate between breeding and molting sites. The northern elephant seal has the longest recorded migration distance of any mammal, at Template:Convert/–Template:Convert/test/Aon. Phocids tend to migrate more than otariids. Travelling pinnipeds may use various features of their environment to reach their destination including geomagnetic fields, water and wind currents, the position of the sun and moon and the taste and temperature of the water.
The rest of the year (late summer and fall), walruses tend to form massive aggregations of tens of thousands of individuals on rocky beaches or outcrops. The migration between the ice and the beach can be long-distance and dramatic. In late spring and summer, for example, several hundred thousand Pacific walruses migrate from the Bering Sea into the Chukchi Sea through the relatively narrow Bering Strait.
Wildebeest are famous for their annual long-distance migration, seemingly timed to coincide with the annual pattern of rainfall and grass growth. The timing of their migrations in both the rainy and dry seasons can vary considerably (by months) from year to year. At the end of the rainy season (May or June in East Africa), wildebeest migrate to dry-season areas in response to a lack of surface (drinking) water. When the rainy season begins again (months later), animals quickly move back to their wet-season ranges. Factors suspected to affect migration include food abundance, surface water availability, predators and phosphorus content in grasses. Phosphorus is a crucial element for all life forms, particularly for lactating female bovids. As a result during the rainy season, wildebeest select grazing areas that contain particularly high phosphorus levels. One study found, in addition to phosphorus, wildebeest select ranges containing grass with relatively high nitrogen content. Large-scale wildebeest migration is quite likely a consequence of decisions being made by individuals at multiple spatial scales, involving a balance of food abundance, food quality, local density of other wildebeest, social interactions, surface water, perceived predation risk, and culturally (or possibly genetically) learned routes and ranges.
Numerous documentaries feature wildebeest crossing rivers, with many being eaten by crocodiles or drowning in the attempt. While having the appearance of a frenzy, recent research has shown a herd of wildebeest possesses what is known as a "swarm intelligence", whereby the animals systematically explore and overcome the obstacle as one. Major predators that feed on wildebeest include the lion, hyena, cheetah, leopard, and crocodile, which seem to favour the wildebeest. Wildebeest, however, are very strong, and can inflict considerable injury even to a lion. Wildebeest have a maximum running speed of around Template:Convert/km/hTemplate:Convert/test/Aon. The primary defensive tactic is herding, where the young animals are protected by the older, larger ones, while the herd runs as a group. Typically, the predators attempt to cut out a young or ill animal and attack without having to worry about the herd. Wildebeest have developed additional sophisticated cooperative behaviours, such as animals taking turns sleeping while others stand guard against a night attack by invading predators. Scientists are unsure how much is learned behaviour and how much is instinct. Wildebeest migrations are closely followed by vultures, as wildebeest carcasses are an important source of food for these scavengers. The vultures consume about 70% of the wildebeest carcasses available. Decreases in the number of migrating wildebeest have also had a negative effect on the vultures. In the Serengeti ecosystem, Tanzania, wildebeest may help facilitate the migration of other, smaller-bodied grazers, such as Thomson's gazelles (Eudorcas thomsonii), which eat the new-growth grasses stimulated by wildebeest foraging.
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